Polyethylene (PE) is synthesized by polymerizing ethylene (CH2═CH2) monomers. Because it is cheap, safe, stable to most environments and easy to be processed, polyethylene polymers are useful in many applications. Polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), MDPE (Medium Density Polyethylene), HDPE (High Density Polyethylene) and LLDPE (Linear Low Density Polyethylene) as well as High Molecular Weight (HMW), Medium Molecular Weight (MMW) and Low Molecular Weight (LMW). Each type of polyethylene has different properties and characteristics.
Ethylene polymerizations are frequently carried out in a loop reactor using ethylene monomer, liquid diluent and catalyst, optionally one or more co-monomer(s) and optionally hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in the form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash vessel, where most of the diluent and unreacted monomers are flashed off and recycled.
Alternatively, the product slurry may be fed to a second loop reactor serially connected to the first loop reactor wherein a second polymer fraction may be produced. Typically, when two reactors in series are employed in this manner, the resultant polymer product is a bimodal polymer product, which comprises a first polymer fraction produced in the first reactor and a second polymer fraction produced in the second reactor, and has a bimodal molecular weight distribution.
After the polymer product is collected from the reactor and the hydrocarbon residues are removed, the polymer product is dried, additives can be added and finally the polymer may be extruded and pelletized.
An important objective when preparing polyethylene and/or copolymers thereof is the production of high-quality polymers having physical properties within certain specifications. However, polyethylene products often show undesirable characteristics such as unacceptable high levels of gels. Furthermore, bimodal polyethylene resins prepared with metallocene catalysts sometimes lead to finished products with visible defects. In particular, dots or specks and/or rough patches are visible on the surface of end-products such as tubes or pipes made from polyethylene products that were produced using a metallocene catalyst. Such defects can make the pipe weaker and can affect the free flow of liquid through the pipe.
In view of the above, there remains a need in the art for polyethylene products with improved properties such as good homogeneity. In particular, there remain a need in the art for polyethylene products with improved properties such as good homogeneity while ensuring low production costs and high-quality end-products.